igneous
Gabbro
Gabbro (mafic intrusive igneous rock)
Hardness: 6-7 (Mohs), Color: Dark green, grey, or black with speckled textures, Luster: Dull to sub-vitreous, Crystal Structure: Phaneritic (coarse-grained), Specific Gravity: 2.7-3.3
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Physical properties
Hardness: 6-7 (Mohs), Color: Dark green, grey, or black with speckled textures, Luster: Dull to sub-vitreous, Crystal Structure: Phaneritic (coarse-grained), Specific Gravity: 2.7-3.3
Formation & geological history
Formed by the slow cooling of magnesium-rich and iron-rich magma inside the Earth's crust. It is the intrusive equivalent of basalt and is often found in oceanic crust and mid-ocean ridges.
Uses & applications
Used as crushed stone for road construction, railroad ballast, and as polished 'black granite' for countertops, floor tiles, and cemetery markers.
Geological facts
Gabbro is the most abundant rock in the deep oceanic crust. It is chemically identical to basalt, but basalt is fine-grained because it cools quickly on the surface.
Field identification & locations
Identified in the field by its dark color and visible interlocking crystals (phaneritic texture) that are roughly equal in size. Common in mountainous regions with exposed deep-crustal or oceanic plates.
